Polyaspartic acid has become increasingly important is recent years as a useful polymer in such areas as water treatment, detergents, cosmetics and hair treatment. In the usual thermal condensation processes to produce the polymer there is produced an initial polysuccinimide which is then hydrolyzed to produce a water soluble salt such a sodium polyaspartate. It has been often noted in the literature that the color of the polysuccinimide is related in rough manner to the time/temperature relationship in its production. For example, the color of the product from processes employing relatively longer reaction time under elevated temperature produces more darkly colored polysuccinimide than is produced With shorter reaction time and lower temperatures. A typical teaching of this phenomenon is found in a publication entitled "Temperature Effect on the Molecular Weight and the Optical Purity of Ahydropolyaspartic acid Prepared by Thermal Polycondensation" by Kokufuta et al., Bulletin of the Chemical Society of Japan, Vol. 51, pp. 1555-1556, 1978.
Many efforts have been made to lower the time/temperature relationship by employing various types of dryers that provide greater efficiency. Recent examples include U.S. Pat. No. 5,057,597, U.S. Pat. No. 5,315,010 and U.S. Pat. No. 5,319,145. In all of these patents dryers are employed in an attempt to convert aspartic acid into polyaspartic acid by thermal condensation. However, the polysuccinimide produces by such processes possess an undesirable color. The color of the polysuccinimide is transferred to the water soluble salt upon hydrolysis of the initial product.
It has been reported in the literature that the use of acidic catalysts such as phosphoric acid reduces the color of the resulting polysuccinimide. In one attempt to overcome the color problem it has been found that the polyaspartate formed by thermal polymerization followed by alkaline hydrolysis is treated with bleach in water solution. This treatment is reported to decolorize the solution of polyaspartate as noted in U.S. Pat. No. 5,292,864 to Wood, et al. However, in many instances the color of the polysuccinimide becomes a disadvantage as this initial product is employed without first converting it to the water soluble polyaspartate salt as was done in the above noted patent to Wood et al. For example, in U.S. Pat. No. 5,266,237 to Freeman et al. and Australian patent AU-A-14775/92, the polysuccinimide is added directly to other ingredients to form a detergent composition. The color of the polysuccinimide, particularly in detergent applications is desirably as colorless as possible. Thus the method of decolorizing the water solution of the salt is not useful in preparing desirable compositions of the above noted patent to Freeman et al. which incorporate the succinimide. Accordingly, there is needed a process for the decolorization of the polysuccinimide.